FR3108457A1 - Conversion chain equipped with damping circuits - Google Patents

Abstract

Conversion chain provided with damping circuits Conversion chain comprising a plurality of conversion modules (12) each comprising a main switch (16) connected in a main electrical line and capable of taking at least one open position in which it blocks circulation an electric current in said main electric line and a closed position in which it permits the flow of an electric current in said main electric line; and a snubber circuit (18) comprising a transformer (20) having a primary winding (12a,12a') and a secondary winding (12b,12b'), connected in series with a semiconductor switching element (26) configured to assume a blocked state in which it prevents the flow of a current in the snubber circuit and at least one on-state in which it allows the flow of an electric current in the snubber circuit between the upper terminal and the lower terminal of the conversion module. Figure for abstract: Fig. 1.

Description

Conversion chain equipped with damping circuits

The present invention relates to the technical field of voltage converters, in particular for high voltage direct current power supply installations (HVDC for "High Voltage Direct Current" in English). The present invention relates more specifically to conversion chains, also called valves, for voltage converters, for example line-commutated converters (LCC) or voltage source converters (VSC for “Voltage Source Converters” in English). These converters can be of the AC/DC, DC/AC or even DC/DC type.

The conversion chains of known voltage converters generally comprise a plurality of conversion modules connected in series in the conversion chain. Traditionally, these conversion modules each comprise a main switch, for example a thyristor, which can be open or closed and a damping circuit forming an auxiliary current path making it possible to at least partially bypass the main switch, in order to adjust the voltage between the terminals of the conversion module, so as to avoid overvoltage at the terminals of the main switch and in order to balance the voltage of the conversion chain between the various conversion modules.

The opening and closing of the main switch of a conversion module of a conversion chain can generate a voltage peak at the terminals of the conversion module which is not desirable. This overvoltage risks in particular damaging the main switch and therefore the converter. In addition, this overvoltage risks generating an imbalance between the voltages at the terminals of the various conversion modules of the conversion chain, so the total voltage of the conversion chain is distributed only over part of the main switches of said conversion chain. . Also, in the event of a voltage imbalance, these main switches are subjected to a voltage higher than their maximum operating voltage, which risks damaging them.

In order to balance the voltages at the terminals of the various conversion modules of a conversion chain, it is known to provide the adjustment devices with a passive RC damping circuit comprising a capacitor and a resistor connected in series (“ RC Snubber” in English). The damping circuit is generally identical for each conversion module. This damping circuit makes it possible to limit the voltage peak at the terminals of the switch in transient state and to balance the voltages between the various conversion modules.

A disadvantage of these passive RC snubber circuits is that they are very large and bulky. In addition, they generate significant energy losses, in particular by the Joule effect, and must generally be associated with a suitable cooling system, for example water. Moreover, these damping circuits cannot be controlled so that they operate optimally only for a part of the operating points of the converter and are inefficient for other operating points.

Conversion chains are also known comprising active damping circuits such as that described in document WO2014/198734. The damping circuit of the conversion chain described in this document comprises a resistor, identical for each of the conversion modules, and a switching element forming an auxiliary switch. This switching element makes it possible to connect the resistor in the conversion module and allows a current to flow in the damping circuit in order to bypass and at least partially short-circuit the main switch. In particular, when the main switch is opened, the switching element is made conductive in order to divert at least part of the current passing through the conversion module towards the damping circuit. This makes it possible to adjust and limit the various reverse recovery currents circulating in the main switches of the conversion modules. In this way, a different reverse recovery current flows through each of the main switches. Also, when the main switch is opened, the switching element is made conductive in order to limit the overvoltage at the terminals of the conversion module and so as to balance the voltages at the terminals of the various conversion modules.

A drawback of this type of conversion chains is that the switching elements are subjected to voltages close to those applied to the main switches and whose values are very high, generally high voltages. These switching elements can thus be subjected to voltages of the order of 6 kilovolts and must therefore be calibrated in voltage so as to withstand such voltages, which can reach 10 kilovolts. They are also crossed by weak currents. This type of switching element, having to withstand high voltage and low current, is very complex to manufacture and particularly expensive and difficult to obtain on the market.

An object of the present invention is to propose a conversion chain for a voltage converter remedying the aforementioned problems.

To do this, the invention relates to a conversion chain for a voltage converter, for example of the high voltage direct current (HVDC) converter type, the conversion chain comprising a plurality of conversion modules connected in series in said chain conversion module, each conversion module having an upper terminal and a lower terminal between which it extends, each conversion module comprising:
- a main electrical line extending between the upper terminal and the lower terminal of said conversion module;
- a main switch connected in said main electric line and able to take at least one open position in which it blocks the flow of an electric current in said main electric line and a closed position in which it authorizes the flow of an electric current in said main power line;
- a damping circuit configured to adjust the voltage between the terminals of the conversion module, connected between the upper terminal and the lower terminal of said conversion module, said damping circuit comprising a transformer having a primary winding and a secondary winding, the primary winding being connected in a primary loop, in series with a capacitor, between the upper terminal and the lower terminal of said converter module, the snubber circuit further comprising a semiconductor switching element connected in series with the secondary winding of the transformer, in a secondary loop, said semiconductor switching element being configured to assume an off state in which it prevents the flow of an electric current in the snubber circuit and at least one on state in which it allows the circulation of an electric current in the damping circuit of the conversion module.

The conversion chain is adapted to be installed in an HVDC voltage converter. Preferably, such a converter can be of the AC/DC or DC/AC type, so that it makes it possible to convert an alternating voltage into a direct voltage, and vice versa, or even of the DC/DC type, so that it converts a first DC voltage into a second DC voltage. In a non-limiting manner, such a voltage converter can be of the VSC or LCC type. Such a conversion chain comprising a plurality of conversion modules is also called a valve.

The main switch is advantageously capable of withstanding high voltages at its terminals, for example voltages greater than 6 kilovolts, generally of the order of 10 kilovolts. The main switch can be a controllable component, whose open or closed position can be controlled, or a passive component, whose open or closed position depends on quantities associated with the conversion chain, such as the circulating current in the main power line.

Each of the snubber circuits is connected in parallel with the corresponding main switch and makes it possible to create an auxiliary current path in which an electric current can flow, when the semiconductor switching element is in the on state. The current flowing in the main line is therefore reduced when the electrical switching element is in said at least one on state. One advantage is in particular to adjust the reverse recovery current flowing in the main switch, which risks appearing when said main switch is opened. The damping circuits notably allow the circulation of a different inverse recovery current in each of the main switches of the conversion chain. This makes it possible to balance the voltage between the terminals of the various conversion modules and therefore to balance the voltage of the conversion chain. The interest is also to reduce the voltage between the upper and lower terminals of each of the conversion modules, in order to avoid an overvoltage.

In each of the conversion modules, the primary loop comprises at least the primary winding of the transformer and the capacitor. The secondary loop includes at least the transformer secondary winding and the semiconductor switching element.

The transformer of each conversion module has a transformation ratio corresponding to the ratio between the number of turns of the secondary winding and the number of turns of the primary winding. As an approximation, the transformation ratio of the transformer is approximately equal to the ratio between the voltage at the terminals of the secondary winding and the voltage at the terminals of the primary winding of the conversion module. This transformation ratio also corresponds, in approximation, to the ratio between the current flowing in the secondary winding and the current flowing in the primary winding. The transformer is sized to allow the desired voltage adaptation.

Thanks to the invention, each transformer makes it possible to adapt the current circulating in the secondary loop with respect to the current circulating in the primary winding of the associated conversion module. Each transformer also makes it possible to adapt, preferably reduce, the voltage across the terminals of the secondary winding with respect to the voltage between the upper and lower terminals of said conversion module, when the semiconductor switching element is in said at least one on state. The voltage at the terminals of the corresponding semiconductor switching element and the electric current passing through it, in the on state, can therefore be adapted, preferably reduced, thanks to the transformer.

The voltage at the terminals of the secondary winding of the transformer of a conversion module is advantageously proportional to the difference between the voltage between the upper and lower terminals of the conversion module and the voltage at the terminals of the capacitor of said conversion module, a ratio substantially equal to the transformation ratio of the transformer.

The transformers of the damping circuits of the conversion chain according to the invention offer an additional degree of freedom in adjusting the voltage at the terminals of the semiconductor switching elements, which reduces the constraints in the choice of these components. . Unlike the devices of the prior art, the voltage rating of the semiconductor switching elements is not imposed by the voltage across the terminals of the corresponding thyristors. On the contrary, thanks to the invention, it is in particular possible to adapt, preferably reduce, the voltage at the terminals of the switching elements with respect to the voltage between the terminals of the conversion modules, according to the voltage ranges allowing a optimal functioning of these switching elements.

Thanks to the invention, the choice and dimensioning of semiconductor switching elements is facilitated. In particular, commercially available semiconductor switching elements can be used. This makes it possible to reduce the costs compared to the devices of the prior art which require the use of specific switching elements, which are particularly expensive, and which must in particular withstand very high voltages.

The use of a transformer in pulse mode, in high power applications such as that of the invention, is not common. The voltage across the terminals of the main switch of a conversion module, when open, has a strong DC component, which cannot be supported by the transformer. The use of a transformer is facilitated by the clever introduction of the capacitor connected in the primary loop. Indeed, this capacitor makes it possible to filter and block this DC component. One interest is to prevent saturation of the transformer. In each of the conversion modules of the conversion chain according to the invention, the semiconductor switching element is preferably maintained in the off state when the main switch of the conversion module is in the closed position. In this way, a current flows in the main line of the conversion module and passes through said main switch, while no current flows in the corresponding damping circuit.

The semiconductor switching element is preferably placed in said at least one on state so as to avoid overvoltage of the main switch. In a non-limiting manner, the semiconductor switching element can be placed in said at least one on state during transient states of the main switch of the corresponding conversion module, or, in other words, when wishes to switch the main switch between the open position and the closed position and vice versa. Indeed, voltage peaks at the terminals of the main switch may appear when the latter is opened and closed. In addition, a reverse recovery current flowing in the opposite direction to the current flowing in the main electrical line risks appearing when the main switch is opened, which can create an imbalance between the voltages of the various conversion modules of the power supply chain. conversion. The semiconductor switching element may be placed in said at least one on state to divert at least a portion of such reverse recovery current to the snubber circuit. This makes it possible to adjust this reverse recovery current flowing in the main line and allows the circulation of a different reverse recovery current in each of the main switches. One interest is to avoid the overvoltage of the main switch. In addition, the balance of the conversion chain tension between the different conversion modules is improved.

For example, just before opening or closing the main switch of a converter module, the semiconductor switching element is placed in an on state. The snubber circuit then creates an auxiliary current path for the current. Preferably, the semiconductor switching element, and in general the snubber circuit, has a low impedance, facilitating the passage of current in the snubber circuit. At least part of the current flowing in the conversion module is then diverted to the snubber circuit. Advantageously, the snubber circuit is configured to at least partially deflect any reverse recovery current that may occur in the main power line. A current then flows in said damping circuit. More precisely, a first current flows in the primary loop and in the primary winding of the transformer and a second current flows in the secondary loop and in the secondary winding of the transformer. On the other hand, the current flowing in the main power line is limited and the voltage at the terminals of the main switch is also limited, thus making it possible to avoid an overvoltage. The main switch is therefore at least partially bypassed, or short-circuited.

The voltage at the terminals of the secondary winding of the transformer, when the semiconductor switching element is closed, then depends on the impedance of the secondary loop, seen by said switching element.

Thanks to the transformer, the voltage at the terminals of the semiconductor switching element differs from the voltage between the terminals of the conversion module, depending on the transformation ratio of the transformer. The main switch can then be placed in the open position without an overvoltage appearing at its terminals during the transient state.

Preferably, all the damping circuits of the conversion chain are substantially identical. In addition, they advantageously have a substantially equal impedance. One interest is to improve the balance between the between the terminals of the conversion modules of the conversion chain, when the semiconductor switching elements are in an on state. The voltages between the terminals of the various conversion modules whose main switches are bypassed are then substantially equal. In other words, the total tension of the conversion chain is evenly distributed between the conversion modules.

Preferably, all the switching elements of the conversion chain are controlled in a similar and simultaneous manner, in order to improve the balance of the voltages between the conversion modules.

In a non-limiting manner, the semiconductor switching element can advantageously be controlled in all-or-nothing mode, between a single off state and a single on state. In the blocked state, its impedance is very large, ideally infinite. In the on state, its impedance is very low. As a variant, the semiconductor switching element can operate in linear mode and be able to assume a plurality of on states, each corresponding to a specific impedance given to the semiconductor switching element.

Advantageously, the snubber circuit may include a resistor connected in series with the secondary winding of the transformer, in the secondary loop. This resistance makes it possible to adjust the current circulating in the secondary loop and the voltage between the upper and lower terminals of the conversion module, this voltage depending on the impedance of the damping circuit.

Preferably, the transformer has a transformation ratio between 0 and 1 between the primary winding and the secondary winding of the transformer. The transformation ratio corresponds to the ratio between the number of turns of the secondary winding and the number of turns of the primary winding. As an approximation, the transformation ratio of the transformer is approximately equal to the ratio between the voltage across the terminals of the secondary winding and the voltage across the terminals of the primary winding of the transformer in operation. One interest is to reduce the voltage across the terminals of the semiconductor switching element compared to the voltage across the terminals of the main switch and therefore between the upper and lower terminals of the conversion module. The semiconductor switching elements of the conversion chain are therefore subjected to a voltage much lower than the voltage to which the switching elements of the devices of the prior art are subjected. Also, it is not necessary to select a semiconductor switching element specifically calibrated in voltage, able to withstand a high voltage, for example greater than 6 kilovolts, which are particularly expensive and not very available on the market. A lower voltage rating switching element may be used. The cost and size of the conversion chain is therefore reduced.

Advantageously, the transformer is an autotransformer, so that the secondary winding is a portion of the primary winding. An advantage is that such an autotransformer has a reduced size compared to conventional transformers. It is also less expensive and has a higher yield.

Advantageously, the main switch is a semiconductor component, for example a thyristor or a diode. If the main switch is a thyristor, then it includes an anode, a cathode and a trigger to control its opening and closing. In which case, the open position of the main switch corresponds to the off state of the thyristor and the closed position corresponds to the on state of the thyristor

The main switch can also be a transistor, for example a bipolar transistor.

Preferably, the damping circuit comprises a diode connected in series with the semiconductor switching element, in the secondary loop. The diode only allows current to flow in one direction, in the secondary loop. An advantage is to impose a substantially zero average voltage across the terminals of the secondary winding of the transformer, in order to avoid its saturation.

Preferably, the semiconductor switching element is a semiconductor component having an operating voltage of less than 4 kilovolts (kV). The semiconductor switching element performs best when subjected to a voltage below 4 kilovolts, and is not likely to be damaged at this voltage. One advantage is that this type of semiconductor switching element is less expensive and bulky than the switching elements calibrated to withstand high voltages. Also, these components are commonly used and can be easily found in the market. The production cost of the conversion chain is therefore reduced and its manufacture is facilitated.

Advantageously, the semiconductor switching element is a transistor, for example an insulated gate bipolar transistor (IGBT) or a metal-oxide gate field effect transistor (MOSFET). The switching on or off of such a transistor can be controlled. It advantageously has a gate, a collector and an emitter, in the case of a bipolar transistor or a gate, a drain and a source in the case of a field effect transistor.

MOSFET transistors are typically configured to operate at low voltage, around 650 volts. IGBT bipolar transistors are traditionally calibrated to withstand higher voltages, between 650 volts and 6.5 kilovolts. The choice of the semiconductor switching element can be made according to the transformer and its transformation ratio.

Advantageously, the semiconductor switching element is capable of assuming a plurality of on states, each on state corresponding to an electric current value passing through said semiconductor switching element. The semiconductor switching element then has a linear operation. Each on state corresponds to a given impedance of said semiconductor switching element. The on state among the plurality of on states in which the semiconductor switching element is placed determines its impedance and therefore the current passing through said semiconductor switching element and circulating in the secondary loop.

The semiconductor switching element therefore makes it possible to control the value of the current flowing in the secondary loop and indirectly the value of the current flowing in the primary loop. One interest is in particular to adjust the reverse recovery current flowing in the main switch in order to avoid an overvoltage of the main switch. The semiconductor switching element also makes it possible to control the voltage across the primary and secondary windings of the transformer and therefore the voltage between the terminals of the conversion module.

This linear operation of the semiconductor switching element provides greater freedom in adjusting the voltage between the terminals of the converter module, compared to a semiconductor switching element operating in an all-or-one mode. Nothing.

Preferably, the conversion chain comprises a control module configured to control the setting to said at least one on state or in the off state of the semiconductor switching element. Switching to an on state is generally ordered before placing the main switch in the open position.

Advantageously, the control module is configured to control the placing in one of the on states of said semiconductor switching element, as a function of a voltage setpoint at the terminals of the secondary winding of the transformer and/or of a current setpoint passing through said secondary winding of the transformer. One interest is to be able to regulate the current circulating in the secondary loop and the voltage at the terminals of the secondary winding, in order to adjust the voltage between the terminals of the conversion module.

Preferably, the main switch comprises two terminals between which it extends, the control module being configured to control passage from the off state of the semiconductor switching element to said at least one on state of said switching element semiconductor when the voltage across the terminals of the main switch exceeds a predetermined voltage threshold. One interest is to avoid damage to the semiconductor switching element in the event of an overvoltage. The voltage threshold depends on the properties of the main switch and is advantageously set at a value lower than the maximum operating voltage of said main switch, in order to guarantee its safety.

In a non-limiting way, the exceeding of the voltage threshold can occur when the main switch is opened or closed.

Advantageously, the control module is configured to control the opening and closing of the main switch.

The invention also relates to a voltage converter for an HVDC installation comprising at least one conversion chain as described previously. This converter can be of the LCC or VSC type and can be an AC/DC converter, a DC/AC converter or even a DC/DC converter.

The invention also relates to a method for controlling a conversion chain as described previously, the method comprising a step of placing said at least one on state of said semiconductor switching element, so as to authorize the circulation of an electric current in the snubber circuit, in order to adjust the voltage between the terminals of the conversion module.

Preferably, the semiconductor switching element is placed in said at least one on state when the voltage across the terminals of the main switch exceeds a predetermined voltage threshold.

Preferably, the semiconductor switching element is placed in an on state chosen from among a plurality of on states, so as to adjust the electric current passing through said semiconductor switching element

This mode of implementation of the method corresponds to a linear control of the semiconductor switching element.

The invention will be better understood on reading the following description of an embodiment of the invention given by way of non-limiting example, with reference to the appended drawings, in which:

FIG. 1 illustrates a conversion chain according to the invention;

FIG. 2 illustrates a first embodiment of a conversion module of the conversion chain of FIG. 1;

FIG. 3 illustrates a second embodiment of a conversion module of the conversion chain of FIG. 1;

FIG. 4 illustrates a third embodiment of a conversion module of the conversion chain of FIG. 1;

FIG. 5 illustrates a fourth embodiment of a conversion module of the conversion chain of FIG. 1; And

FIG. 6 illustrates an AC/DC voltage converter comprising a plurality of conversion chains according to the invention.

The invention relates to a conversion chain for a voltage converter, in particular an HVDC voltage converter.

In a non-limiting manner, the voltage converter can be of the AC/DC type and make it possible to convert an alternating voltage into a direct voltage and vice versa. As a variant, and still in a non-limiting way, the voltage converter can be of the DC/DC type and make it possible to convert a first DC voltage into a second DC voltage and vice versa. The voltage converter can also be a line commutated converter (LCC) or a voltage source converter (VSC).

FIG. 1 illustrates a conversion chain 10 according to the invention for such a voltage converter. The conversion chain 10 comprises a plurality of conversion modules 12 connected in series with each other in the conversion chain 10 . In this non-limiting example, the conversion chain 10 comprises three conversion modules 12 . Each of the conversion modules 12 comprises an upper terminal 12a and a lower terminal 12b between which it extends. The lower terminal 12b of a conversion module 12 is electrically connected to the upper terminal 12a of the directly adjacent conversion module 12 .

Each of the conversion modules 12 comprises an electrically main line 14 extending between its upper terminal 12a and its lower terminal 12b . Each conversion module 12 comprises a main switch 16 connected in its main line 14 between the upper terminal 12a and the lower terminal 12b . These main switches can assume an open position and a closed position.

In this non-limiting example, the main switches16are semiconductor switches and more precisely thyristors. They each include an anodeAT connected electrically to the upper terminal12aof the corresponding conversion module, a cathodeKelectrically connected to the lower terminal12bof the corresponding conversion module, and a triggerG To for controlling the thyristor. Each main switch16can assume an open position, corresponding to the blocked state of the thyristor, in which it blocks the flow of an electric current in said main electric line14, for example if the voltage between the anodeATand the cathodeKis negative or if this voltage is positive but the incoming current on the triggerG Tois zero. Each main switch16can also assume a closed position, corresponding to the on state of the thyristor, in which it authorizes the circulation of an electric current in said main electric line14, especially if the voltage between the anodeATand the cathodeKis positive and a positive current pulse has been applied to the triggerG To.

In this non-limiting example, all the main switches 16 are substantially identical and of the same type.

Each conversion module 12 also comprises a damping circuit 18 configured to adjust the voltage between the terminals of the conversion module, connected between the upper and lower terminals 12a , 12b of the corresponding conversion module 12 . Each damping circuit 18 is connected in parallel with the main switch 16 of the conversion module.

FIG. 2 illustrates a first embodiment of a conversion module 12 of the conversion chain 10 of FIG. 1 . The main switch 16 has a voltage V _T at its terminals, and therefore between the upper and lower terminals 12a , 12b . The damping circuit 18 of the conversion module 12 comprises a transformer 20 . In this non-limiting example, the anode of thyristor 16 is connected to the lower terminal 12b of the conversion module and its cathode is connected to the upper terminal 12a of the conversion module.

In this first non-limiting embodiment, the transformer 20 is of the single-phase type, so that it comprises a primary winding 20a and a secondary winding 20b , galvanically isolated. The transformer is normally polarized.

The primary winding 20a of the transformer 20 is connected in a primary loop 22 between the upper terminal 12a and the lower terminal 12b of the conversion module 12 . A capacitor 24 is connected in series with the primary winding 20a of the transformer 20 , in the primary loop 22 , between the upper terminal 12a of the conversion module and said primary winding.

Snubber circuit 18 further includes a semiconductor switching element 26 connected in series with secondary winding 20b of transformer 20 . The secondary winding 20b is connected in a secondary loop 28 . A diode 30 is also connected in series in the secondary loop 28 , between the secondary winding 20b and the semiconductor switching element 26 . The diode conducts in the direction from the secondary winding to the semiconductor switching element.

The primary winding 20a of the transformer 20 has a voltage V1 at its terminals while the secondary winding 20b has a voltage V2 at its terminals. The transformer has a transformation ratio between its primary winding 20a and its secondary winding 20b which is, in this non-limiting example, less than 1 and between 0 and 1. The voltage across the terminals of the secondary winding 20b is therefore strictly lower to the voltage across the terminals of the primary winding 20a , in operation.

Without departing from the scope of the invention, thyristor 16 could be reversed so as to conduct the current in the opposite direction. The direction of the diode and the polarity of the transformer should then be adapted.

The transformers of the conversion chain according to the invention offer an additional degree of freedom in adjusting the voltage at the terminals of the semiconductor switching elements 26 , which reduces the constraints in the choice of these components. Unlike the devices of the prior art, the size of the semiconductor switching elements is not imposed by the voltage across the terminals of the corresponding main switches. On the contrary, thanks to the invention, it is in particular possible to adapt the voltage at the terminals of the semiconductor switching elements according to the voltage ranges allowing optimum operation of these semiconductor switching elements.

In this non-limiting example, the semiconductor switching element 26 is a metal-oxide gate field effect transistor (MOSFET) so that it includes a gate G , a drain D and a source S. This semiconductor switching element 26 can assume a blocked state in which it behaves like an open switch and it does not allow the circulation of an electric current in the secondary loop 28 and in the secondary winding 20b of the transformer 20 and therefore in the damping circuit 18 . Without departing from the scope of the invention, the semiconductor switching element could be an IGBT transistor.

In this non-limiting example, the semiconductor switching element 26 can also assume a plurality of on states in which it authorizes the circulation of an electric current in the damping circuit 18 and more precisely of an electric current i2 in the secondary loop 28 and in the secondary winding 20b of the transformer. In each of these on states, the semiconductor switching element 26 behaves like a closed switch, so that the snubber circuit forms an auxiliary current path allowing current to be diverted. The current flowing in the main electric line 14 is then reduced. The impedance of the semiconductor switching element 26 is different for each of the on states, so that the electric current i2 flowing in the secondary loop 28 can be regulated by controlling the semiconductor switching element 26 and by placing it in one of the on states. The semiconductor switching element therefore makes it possible to regulate the electric current i 2 flowing in the secondary loop and the voltage V 2 at the terminals of the secondary winding. The semiconductor switching element 26 can therefore be controlled linearly. Alternatively, the semiconductor switching element could be controlled in all-or-nothing operation, so that it would only switch between an off state and a single on state.

As illustrated in FIG. 2 , the conversion chain 10 also comprises a control module 40 making it possible to control the main switches 16 and the semiconductor switching elements 26 of the conversion modules 12 . In particular, the control module 40 is configured to control the switching on or off of the semiconductor switching element 26 . In this non-limiting example, it also makes it possible to control the semiconductor switching element 26 by adjusting the voltage V _GS between the gate and the source of the semiconductor switching element according to a setpoint of voltage V _T * between the upper and lower terminals of the conversion module 12 .

A mode of implementation of the method of the conversion chain 10 , in transient state, by means of the control module 40 , will now be detailed, and more precisely the control of the conversion module of FIG . the overvoltage that may appear when the main switch 16 is opened.

In a non-limiting manner, all of the conversion modules 10 , and in particular of the semiconductor switching elements 26 are controlled in a similar manner and simultaneously, for example by identical electronic cards.

In steady state, the main switch 16 of the conversion module 12 is in the closed position and is crossed by a current flowing in the main line 14 . It has a voltage V _T across its terminals which is a high voltage, approximately equal to 10 kilovolts. The semiconductor switching element 26 is in an off state, so that no current flows through the snubber circuit 18 .

When it is intended to place the main switch 16 in the open position, the semiconductor switching element 26 is placed in an on state, so that a current flows in the snubber circuit 18 . The damping circuit 18 then generates an auxiliary path for the circulation of the electric current, so that the current flowing in the conversion module is at least partially diverted towards the damping circuit. 18 . The current flowing in the main power line 14 , through the main switch 16 , is reduced. This makes it possible in particular to adjust the reverse recovery current flowing in the main switch 16 . Thanks to the damping circuit 18 , the voltage between the terminals of the main switch is therefore limited, which makes it possible to avoid an overvoltage of the main switch.

A current i2 then flows in the secondary loop 28 and the secondary winding 20b has a voltage V2 at its terminals, while the primary winding 20a has a voltage V1 , greater than V2 , at its terminals. Diode 30 prevents the flow of a reverse current in the secondary loop. Capacitor 24 filters and blocks the DC component of voltage V _T at the terminals of main switch 16 in order to avoid saturation of transformer 20 .

As a variant, the semiconductor switching element 26 could be placed in an on state when the voltage V _T across the terminals of the main switch 16 exceeds a predetermined voltage threshold, for example a threshold slightly lower than the maximum voltage of operation of said main switch.

According to the invention, thanks to the transformer 20 , the voltage to which the semiconductor switching element 26 is subjected, in the on state, is adapted and reduced with respect to the voltage V _T at the terminals of the main switch 16 . The semiconductor switching element 26 is subjected to a voltage having an order of magnitude close to the voltage V2 at the terminals of the secondary winding 20b of the transformer, which is in this example much lower than the voltage V _T at the terminals of the main switch 16 . The transformer makes it possible to use a MOSFET transistor as a semiconductor switching element, having optimum operation at very low voltage, at approximately 650 volts. The transformer 20 makes it possible to dispense with the use of a semiconductor switching element of high voltage caliber, capable of withstanding a very high voltage, of more than 4 kilovolts. This makes it possible to reduce the costs compared with the devices of the prior art which require the use of specific switching elements, which are particularly costly, and which must in particular withstand very high voltages.

The main switch16can then be opened without an overvoltage appearing between the upper terminals12aand lower12b of the conversion module. One interest is to avoid damaging the conversion module12and more generally of the voltage converter. Damping circuits18balance the voltages between the upper and lower terminals12a,12bdifferent conversion modules12 of the conversion chain10.

In a non-limiting manner, the damping circuits 18 of all the conversion modules are similar and have a substantially identical impedance in order to improve the balance between the voltages between the terminals of the conversion modules 12 , when the elements switching semiconductors are in the on state.

In this non-limiting example, the voltage V _GS between the gate and the source of transistor 26 can be controlled using the control module, so as to place the semiconductor switching element in an on state chosen from among the plurality of possible on-states. This makes it possible to regulate the current i2 circulating in the secondary loop 28 and the voltage V2 at the terminals of the secondary winding 20b of the transformer 20 , in particular in order to operate in an optimal operating range of the semiconductor switching element and not not exceed its current and voltage operating thresholds.

The semiconductor switching element can then be placed again in the off state, so as to interrupt the flow of a current in the damping circuit 18 of the conversion module 12 .

FIG. 3 illustrates a second embodiment of a conversion module 12 of a conversion chain 10 according to the invention. In this second embodiment, the transformer 20 is reverse biased. Further, thyristor 16 is connected in the other direction, so that its anode is connected to the upper terminal 12a of the conversion module and its cathode is connected to the lower terminal 12b .

FIG. 4 illustrates a third embodiment of a conversion module 12 of a conversion chain 10 according to the invention. In this third embodiment, the transformer 20' is an autotransformer so that the secondary winding 20b' is a portion of the primary winding 20a' .

Again, the thyristor16is connected so that its anode is connected to the upper terminal12aof the conversion module and its cathode is connected to the lower terminal12bof the conversion module12. The diode30is connected between the drain of the semiconductor switching element26 and the secondary winding20b'of the transformer20'. The diode connected so as to be conductive in the direction going from the drain of the semiconductor switching element26to the secondary winding20b'.

FIG. 5 illustrates a fourth embodiment of a conversion module 12 of a conversion chain 10 according to the invention. In this third embodiment, transformer 20' is also an autotransformer.

The thyristor16is connected so that its anode is connected to the lower terminal12bof the conversion module and its cathode is connected to the upper terminal12aof the conversion module12. In this non-limiting example, the capacitor24is connected between the bottom terminal12bof the conversion module and the primary winding20a ' oftransformer20'.

FIG. 6 illustrates an AC/DC voltage converter 100 comprising a plurality of conversion chains 10 according to the invention. The voltage converter 100 is of the LCC type and makes it possible to convert an alternating voltage into a direct voltage. It also makes it possible to connect an AC power supply network and a DC power supply network to one another. Such a voltage converter 100 is particularly suitable for being installed in an HVDC installation.

In this non-limiting example, the voltage converter 100 comprises an upper conversion device 102 connected to a lower conversion device 104 . Each conversion device 102 , 104 comprises three arms 106 each comprising two conversion chains 10 according to the invention. In each arm 106 , the conversion chains 10 are connected together at an intermediate point 110 . The arms of the conversion device 102 , 104 are connected to two transformers 108 .

Claims (16)

Conversion chain (10) for a voltage converter, for example of the high voltage direct current (HVDC) converter type, the conversion chain comprising a plurality of conversion modules (12) connected in series in said conversion chain, each conversion module (12) having an upper terminal (12a) and a lower terminal (12b) between which it extends, each conversion module comprising:
- a main electrical line (14) extending between the upper terminal and the lower terminal of said conversion module;
- a main switch (16) connected in said main electric line and able to assume at least one open position in which it blocks the flow of an electric current in said main electric line and a closed position in which it authorizes the flow of a electric current in said main electric line;
- a snubber circuit (18) configured to adjust the voltage between the terminals of the converter module, connected between the upper terminal and the lower terminal of said converter module, said snubber circuit comprising a transformer (20) having a winding primary (12a, 12a') and a secondary winding (12b, 12b'), the primary winding being connected in a primary loop (22), in series with a capacitor (24), between the upper terminal and the lower terminal of said conversion module, the snubber circuit further comprising a semiconductor switching element (26) connected in series with the secondary winding of the transformer, in a secondary loop (28), said semiconductor switching element being configured to take a blocked state in which it prevents the flow of a current in the damping circuit and at least one on state in which it allows the flow of an electric current in the damping circuit of the conversion module.
Conversion chain according to claim 1 , in which the transformer (20,20') has a transformation ratio of between 0 and 1 between the primary winding (12a,12a') and the secondary winding (12b,12b') of the transformer. Conversion chain according to Claim 1 or 2 , in which the transformer (20') is an autotransformer, so that the secondary winding (20b') is a portion of the primary winding (20a'). Conversion chain according to any one of Claims 1 to 3 , in which the main switch (16) is a semiconductor component, for example a thyristor or a diode. Conversion chain according to any one of Claims 1 to 4 , in which the damping circuit (18) comprises a diode connected in series with the semiconductor switching element, in the secondary loop. Conversion chain according to any one of Claims 1 to 5 , in which the damping circuit (18) comprises a resistor connected in series with the semiconductor switching element, in the secondary loop (28). Conversion chain according to any one of Claims 1 to 6 , in which the semiconductor switching element (26) is a semiconductor component having an operating voltage of less than 4 kilovolts (kV). Conversion chain according to any one of Claims 1 to 7 , in which the semiconductor switching element (26) is a transistor, for example an insulated-gate transistor (IGBT) or a gate field-effect transistor metal-oxide (MOSFET). Conversion chain according to any one of claims 1 to 8 , in which the semiconductor switching element (26) is capable of assuming a plurality of on states, each on state corresponding to an electric current value passing through said semiconductor switching element. Conversion chain according to any one of claims 1 to 9 , further comprising a control module configured to control the setting to said at least one on state or in the off state of the semiconductor switching element (26) . Conversion chain according to Claims 9 and 10 , in which the control module is configured to control the placing in one of the on states of the said semiconductor switching element, as a function of a voltage setpoint at the terminals of the secondary winding (20b, 20b') of the transformer (20) and/or of a current set point passing through said secondary winding of the transformer. Conversion chain according to claim 10 or 11 , in which the main switch (16) comprises two terminals between which it extends, and in which the control module is configured to control the passage from the blocked state of the semiconductor switching element (26) to said at least one on state of said semiconductor switching element when the voltage across the main switch exceeds a predetermined voltage threshold. Voltage converter for an HVDC installation, comprising at least one conversion chain (10) according to any one of Claims 1 to 12 . Method for controlling a conversion chain according to any one of claims 1 to 12 , comprising a step of placing said at least one on state of said semiconductor switching element (26) in such a way as to authorize the circulation of an electric current in the snubber circuit (18) to adjust the voltage between the terminals of the conversion module. Control method according to claim 14 , in which the semiconductor switching element (26) is placed in said at least one on state when the voltage across the terminals of the main switch (26) exceeds a predetermined voltage threshold. Control method according to claim 14 or 15 , in which the semiconductor switching element (26) is placed in an on state chosen from among a plurality of on states, so as to adjust the electric current passing through said switching element semiconductor.